A. Technical Field
The present invention relates to a particulate water-absorbent resin composition. More specifically, the present invention relates to a particulate water-absorbent resin composition which is so excellent as to be able to exhibit a high absorption capacity over a long period when used for sanitary materials such as disposable diapers, sanitary napkins, and incontinent pads.
B. Background Art
At present, water-absorbent resins, their compositions, and hydrophilic fibers (e.g. pulp) are widely used for sanitary materials such as disposable diapers, sanitary napkins, and so-called incontinent pads as their component materials for the purpose of absorption of body fluids. Examples of materials used as main raw materials for the above water-absorbent resins include: partially-neutralized and crosslinked poly(acrylic acids); hydrolyzed graft polymers of starch-acrylic acid; saponified copolymers of vinyl acetate-acrylic acid esters; hydrolyzed copolymers of acrylonitrile or acrylamide, or crosslinked polymers of these hydrolyzed copolymers; and crosslinked polymers of cationic monomers.
In recent years, as to the sanitary materials such as disposable diapers and sanitary napkins, their high functionalization and thinning are making progress, so there is a tendency toward increases in the amount of the water-absorbent resin as used per piece of sanitary material and in the mass ratio of the water-absorbent resin relative to a whole absorbent structure consisting of such as the water-absorbent resin and the hydrophilic fibers. Specifically, the content of the water-absorbent resin in the absorbent structure is raised by decreasing the amount of the hydrophilic fibers (which have a small bulk density) and increasing the amount of the water-absorbent resin (which has excellent water absorbency and a large bulk density) as used. Thereby the thinning of the sanitary materials is aimed at without lowering the water absorption quantity.
When the performances of the water-absorbent resin are evaluated, commonly, the evaluations have hitherto been carried out by such as absorption capacity without load, absorption capacity under load, and absorption index under load (e.g. refer to patent document 1 below).
In conventional methods for evaluation of the water-absorbent resin, generally, it has been common that the absorption capacity in 1 hour from the absorption initiation is measured. Its reason is that conventional water-absorbent resins almost reach no less than their saturated absorption capacities in 1 hour from the absorption initiation.
However, if attention is directed to practical use for diapers, then the diapers are worn for a period of several hours and, in this wearing period, urine is discharged several times at intervals of about tens of minutes to about 1 hour.
Accordingly, as properties of water-absorbent resins suitable for uses such as diapers, it is demanded that: the saturation is not reached in 1 hour, but the absorption ability in an initial short time is secured in some degree, and further, the absorption capacity lastingly increases over a long period, and also the liquid permeability and the liquid diffusibility can be secured. Particularly, in recent years when the thinning makes progress, it is demanded that a high absorption capacity under load can be exhibited over a long period.
Almost all conventional water-absorbent resins almost reach no less than their saturated absorption capacities in 1 hour as mentioned above. Among them, there is a water-absorbent resin which exhibits an extremely low absorption capacity under load. Because such a water-absorbent resin is weak in gel strength and can secure neither the liquid permeability nor the liquid diffusibility, it has had problems when used for such as diapers. It is well known that: when practically used for such as diapers, such a water-absorbent resin swells only in the neighborhood of the urination portion to thus cause gel-blocking, so that it becomes impossible to diffuse urine all over the absorbent structure, thus resulting in inferior absorption performances. This is remarkable particularly as to such a high-concentration absorbent structure as has a water-absorbent resin concentration of higher than 40% or 50% in the absorbent structure.
In addition, it is commonly known that: the lower absorption capacity without load the water-absorbent resin exhibits, the higher its liquid permeability and liquid diffusibility are. However, there has been a problem that, in the case where a water-absorbent resin which exhibits a low absorption capacity is used for practical diapers, its ability to take a liquid into the diapers (liquid permeability) is high, but the absorption quantities of the diapers unfavorably decrease.
A way of making the absorption capacity increase lastingly over a long period is to enlarge the particle diameters of the water-absorbent resin. However, there has been a problem that: if the particle diameters are enlarged, then the use for diapers involves the texture deterioration of diaper surfaces to thus give users a foreign substance feeling.
There are reported some arts in which a mixed-bed type ion-exchange resin is applied in order to make the absorption capacity increase lastingly over a long period.
There is reported a superabsorbent material including a combination of an anionic superabsorbent material in which 20 to 100% of functional groups are free-acid types and an anion exchanger in which 20 to 100% of functional groups are basic types (e.g. refer to patent document 2 below).
There is reported a composition which is a mixed-bed type ion-exchange water-absorbent resin composed of a mixture of a free-functional-group-containing water-swellable anion exchanger and a free-functional-group-containing water-swellable cation exchanger and is characterized in that the absorption capacity under load (0.7 psi (4.83 kPa)) PUP (Performance Under Pressure) is not less than 30 g/g in 2 hours or not less than 40 g/g in 8 hours or not less than 42 g/g in 16 hours (e.g. refer to patent document 3 below).
There is reported a water-absorbent resin in which a microdomain is formed in one particle by a water-absorbent resin in which functional groups exhibiting the basicity exist in a free state and a water-absorbent resin in which functional groups exhibiting the acidity exist in a free state (e.g. refer to patent document 4 below).
All in the above arts, the technology of the mixed-bed type ion-exchange resin is applied to water-absorbent resins, and salts dissolved in urine are taken into functional groups on the water-absorbent resin by ion exchange, whereby the salt concentration of the urine to be absorbed is reduced, and also the osmotic pressure is generated by the dissociation of ions from the functional groups, so that a higher absorption capacity is exhibited. Accordingly, in the above arts, the deceleration of the absorption rate is achieved by adjusting the ion-exchange rate.
However, the above arts have a demerit such that: in the case where the resin is exposed to an aqueous liquid having a salt concentration more than the ion-exchange capacity, the ion exchange occurs, but it does not attain the reduction of such a high salt concentration, so that the swelling capacity sharply deteriorates. In addition, many of the cationic superabsorbent materials (anion exchangers) are extremely expensive, and it has therefore been difficult to provide an inexpensive water-absorbent resin.
In addition, there is also reported an art in which fine particles of poly(acrylic acid) (corresponding to not larger than 45 μm) are added to water-absorbent resin particles to thereby control the liquid permeability (e.g. refer to patent document 5 below). Furthermore, as an art for obtaining a water-absorbing agent by mixing water-absorbent resin particles having different neutralization degrees, such an art is reported also for the purpose of deodorization of such as ammonia (e.g. refer to patent document 6 below). However, this art has a demerit such that: the absorption capacity under load is low, and also, there is almost no effect or only a small effect of increasing the absorption capacity over a long period.                [Patent Document 1] U.S. Pat. No. 5,601,542        [Patent Document 2] Pamphlet of WO 96/15180        [Patent Document 3] Pamphlet of WO 99/34843        [Patent Document 4] Pamphlet of WO 99/25393        [Patent Document 5] JP-A-098170/2001 (Kokai)        [Patent Document 6] Pamphlet of WO 03/28778        